Olefins are a class of hydrocarbons with a single double bond and a chemical formula of CnH2n. Ethylene and propylene are important industrially-produced olefins, because they feature a highly reactive double bond and provide an ideal molecule for conversion to many useful products. For example, olefins are used for the production of polymers such as polyethylene and polypropylene. Other olefin-based petrochemical products include ethylene dichloride, ethylene oxide, propylene oxide, oxo alcohol, polystyrene, and acrylonitrile.
Most of the world's ethylene and propylene is produced from thermal cracking of hydrocarbon feedstocks ranging from ethane to vacuum gas oils. There are a number of proprietary technologies for the production of olefins in a cracking furnace including, for example, Selective Cracking Optimum Recovery (SCORE™). This technology employs a pyrolysis furnace designed for reaction of the feed stream with high olefin yield and a low-pressure distillation tower designed for efficient product recovery. A typical cracking furnace is a large complex machine that produces a large quantity of olefin product, for example, 100,000 metric tons per annum (100 kTA), 220,000 metric tons per annum (220 kTA), or more.
In a cracking furnace, gaseous or liquid hydrocarbon feed such as naphtha or ethane is quickly heated in the absence of oxygen. Reaction occurs at a very high temperature, for example, from 1450° F. to 1700° F., for a very short period of time. Residence time may be on the order of milliseconds and is kept low by the use of high gas velocities, thereby improving product yield.
Normal operation of olefin cracking furnaces results in a gradual build-up of coke within the furnace tubes. Decoking is the process of periodically removing built-up coke from the interior of the furnace tubes in order to maintain heat exchange efficiency and proper operation of the furnace.
An example decoking process is steam-air decoking Steam-air decoking may be conducted periodically and does not require performing a cold shutdown of the furnace, which reduces the time needed to get the furnace back online following decoking and is an important productivity consideration. In steam-air decoking, low pressure steam (e.g., 135 psig) and air are introduced into the furnace. Oxygen from the air reacts with the carbon deposits on the interior surfaces of the furnace tubes to form carbon dioxide gas, which is exhausted from the furnace, typically, to atmosphere. The steam serves to control temperature within the furnace during the decoking procedure. Steam-air decoking offers advantages over other decoking procedures such as steam-water decoke in which coke deposits react with water to form carbon monoxide and hydrogen.
Dislodged coke flows out of the furnace with the steam/air exhaust; however, obstructions may occur if dislodged coke accumulates in a pipe or at an outlet. For example, during a decoking procedure, large pieces of coke may dislodge from valleys of internally-finned radiant tubes and accumulate in a downstream elbow joint of a primary transfer line exchange outlet. This could result in low flow of air and steam through the radiant tubes leading to the obstructed outlet. Low flow of steam during decoke can cause temperature build-up in the affected tubes, necessitating a cold furnace shutdown to clear the pluggage. A cold furnace shutdown significantly increases the time required to bring the furnace back to normal operation, reducing productivity. A new technique is needed to address the problem of accumulating coke in an outlet of an olefin cracking furnace during a decoking procedure.